1. Field of the Invention
The present invention relates to a method for synthesizing diamond by using a plasma.
2. Discussion of Background
There have so far been known the following methods for synthesizing diamond in a thermodynamically metastable region by using electric discharge.
(1) An ion beam or ion plating method wherein carbon ions or hydrocarbon ions are produced by using electric discharge, and they are accelerated by a potential gradient and impinged against a substrate surface to precipitate diamond.
(2) An activated gas phase precipitation method wherein a gas mixture comprising a hydrocarbon and hydrogen is activated by a low temperature plasma produced by glow discharge, whereby diamond is precipitated on the surface of a substrate.
However, these methods have the following drawbacks.
Method (1) has an advantage that a diamond-like carbon film can be formed on the substrate surface made of various materials at a normal temperature. However, since accelerated ions are employed the formed diamond has many defects, and it is difficult to obtain diamond having good crystallinity. Further, since the ion beam density can hardly be increased, this method has a disadvantage that the precipitation rate is low.
Method (2) has an advantage that microcrystals of diamond can be formed on the substrate made of various materials. However, since a low temperature plasma produced by glow discharge is employed, it is difficult to generate such a plasma unless the gas pressure is as low as 0.3 atm or less. Further, the concentration of active species such as ions or radicals is as low as about 10% at the maximum. Thus, there has been a drawback that the rate of growth of diamond is slow (a few micrometers per hour at the maximum). The gas (ions, atoms, molecules) temperature in this low temperature plasma is about 1600.degree. K. or lower.
Further, in each of methods (1) and (2), diamond can be formed only on a substrate, and it is impossible to form it in a gas phase in the form of a powder.
In order to overcome the drawbacks of the conventional method, the present inventors have previously developed a method for synthesizing diamond which comprises generating a high temperature plasma having a gas temperature of at least 1700.degree. K. by means of a direct current, a low frequency, a high frequency or a microwave, and decomposing or evaporating an organic compound or a carbon material in the plasma to precipitate diamond, whereby not only a film-form diamond but also a bulk form diamond can be efficiently produced at a high growth rate of diamond (U.S. patent application Ser. No. 06/943,726)
According to this method, diamond can be obtained under high gas pressure at a high concentration of active species and at a high growth rate of diamond. However, due to the high gas temperature of the plasma, it is usually required to cool the substrate or the substrate holder by a cooling medium, or to control the temperature of the precipitation zone by blowing a gas or a cooling medium. Such a temperature control tends to lead to a non-uniformity of the substate temperature or the precipitation zone temperature, and the temperature control is difficult.